1. Field of the Invention
This invention relates generally to thin-film write heads and more particularly to write heads employing alloys with high saturation flux densities for the magnetic poles in an inductive write element that is useful for writing data to high-coercivity magnetic media.
2. Description of the Related Art
The terminology and units used in the magnetic materials arts vary from one region to another. Accordingly, a brief summary of terminology used herein is presented for clarity. Magnetic Flux is expressed in Système International d'unités (SI) units of webers (Wb) or volt-seconds, each of which is exactly equivalent to 100,000,000 maxwells (Mx). Magnetic Flux Density (B) is expressed in SI units of teslas (T), each of which is exactly equivalent to 10,000 gauss (G). Magnetic Field Intensity (H) is expressed in SI units of amperes per meter (A/m), each of which is approximately equivalent to about 0.0126 oersteds (Oe). As used herein, the Permanent Magnetic Moment or Magnetization (BM) of a material is the magnetic flux density (B) in teslas present in the material with no external magnetic H-field applied. The Saturation Flux Density (BS )of a material (commonly denominated 4πMS) is the maximum magnetic flux density (B) in teslas that can be induced in the material by a large external magnetic field (H-field). The Remanence or Retentivity (BR<BS) of a material is the magnetic moment in teslas remaining in the material after forcing the material into saturation along the easy axis and then removing the external H-field. The Coercivity (HC) of a material is the magnetic field (H-field) intensity in amperes per meter required to overcome the remanence moment (BR) to reduce the magnetic flux density (B) in the material to zero along the easy axis. The Anisotropic Field (HK>HC) of a material is the magnetic field (H-field) intensity in amperes per meter required to induce the saturation flux density (BS) in the material along the hard axis normal to the easy axis. The permeability of a material (μ) is defined as the ratio B/H with appropriate units and may be shown to be about the same as BS/HK when large.
The inductive head and the inductive/magnetoresistive (MR) head are well-known in the art. Both of these heads can write and read signals with respect to a magnetic medium such as a rotating disk medium or a streaming tape medium. The inductive head usually includes first and second poles having first and second pole tips, respectively. The pole tips are separated by a gap at an air bearing surface (ABS) or head surface. A coil is disposed between the first and second poles to couple magnetically thereto. The head assembly uses an inductive write head portion to perform write functions and a MR read head portion to perform read functions. The read head portion includes an MR sensor sandwiched between a pair of read gap layers, which are in turn sandwiched between first and second shield layers. Either type of magnetic head is usually mounted on or embedded in a slider that is supported in a transducing relationship with respect to a magnetic medium. The magnetic medium may be either a magnetic disk or a magnetic tape.
Considerable effort has been undertaken by practitioners in the art to increase the recording density of magnetic heads. Decreasing the length (i.e., the thickness) of the gap between the first and second pole tips permits writing of more bits per inch of media. Further, increasing the coercivity (HC) of the magnetic medium allows the medium to accurately retain data with a higher areal bit density with less thermal degradation. A consequence of such higher bit density is a higher data transfer rate for information between the head and the medium.
These magnetic media coercivity and density improvements require the magnetic pole materials to conduct relatively high magnetic flux densities, especially those portions of the poles (the pole tips) adjacent to the gap at the ABS. However, the ferromagnetic (FM) pole materials have a saturation flux density (BS) limit beyond which they can conduct no more magnetic flux. Accordingly, there is a need for a pole tip structure having a high saturation flux density (BS) to operate effectively with newer high-coercivity magnetic media.
The first and second pole pieces, including the pole tips, are commonly constructed of Permalloy (Ni81Fe19), which combines 81% nickel with 19% iron by weight. Permalloy is a desirable material for pole-construction, having good soft magnetic properties (low coercivity HC and high saturation flux density BS) and being easy to shape by normal patterning and deposition techniques. Further, Permalloy has good corrosion resistance for head reliability. Permalloy has a saturation flux density (BS) of about 1.0 T (10 kG) and a coercivity (HC) of no more than 20 A/m (0.2 Oe) at worst. But it is desirable to increase this saturation flux density (BS) value so that the pole tips can carry the larger magnetic flux density required to overcome the high coercivity (HC) of modem recording media without saturating.
Cobalt-based magnetic alloys have a higher saturation flux density (BS) than Permalloy. However, cobalt materials have significantly worse corrosion resistance. Another family of high-BS materials is the sputtered FeNiX materials, where X is from the group of tantalum, aluminum, and rhodium. But sputter-deposition of the pole pieces is not as desirable as frame-plating because ion-milling is required after sputtering to shape the trackwidth of the pole tips. This process is very difficult to implement. And sputtered materials exhibit a high stress that can distort recorded signals. Moreover, magnetically forming a thick film of such materials using sputtering is difficult because the sputtered material has a large magnetocrystalline anisotropy and the crystal grain size of the sputtered film becomes large so the anisotropic field (HK) is disadvantageously large. An electroplating method is preferred to suppress the crystal grain size to a small value to minimize the anisotropic field (HK) while retaining the desired high saturation flux density (BS).
The commonly-assigned U.S. Pat. No. 4,589,042 discloses an inductive read/write head wherein the pole tip regions of the magnetic poles are fabricated of a high-BS nickel-iron alloy material (Ni45Fe55) with about 55% iron by weight, while the remainder of the pole structure is made of Permalloy. But the Ni45Fe55 material exhibits high magnetostriction and can be used only if the head design is modified to accommodate the magnetostrictive characteristics of the material.
The commonly-assigned U.S. Pat. No. 5,864,450 also discloses a head pole structure using a nickel-iron alloy with 50-60% iron by weight. This material has a saturation flux density (BS ) in the range of 1.5 to 1.7 T (15 to 17 kG) with high resistivity and a lower permeability than Permalloy. The saturation flux density (BS) of the pole-tips is further increased by employing a metal-in-gap (MIG) configuration at the pole-tips such that one or more of the pole tips is configured in a bilayer with one of the layers being the higher-BS Ni45Fe55 material and the other layer having a lower BS. The higher-BS Ni45Fe55 material is placed adjacent the gap where it is most needed and the remaining material can be selected to mitigate the magnetostriction problem.
In U.S. Pat. No. 6,262,867, Sano et al. disclose an electroplated thin film pole structure made of a nickel-iron alloy having 38% to 60% nickel by weight and 40% to 62% iron by weight with a crystal grain size smaller than 50 nanometers. Sano et al. teach that the saturation flux density (BS) drops below 1.5 T (15 kG) when the proportion of nickel in the alloy drops below 38% or rises above 60% and suggest adding one element selected from the group consisting of cobalt, molybdenum, chromium and palladium in an amount less than 3% by weight.
In U.S. Pat. No. 5,372,698, Liao discloses an electroplated pole structure that includes 90% cobalt and a trace of boron with iron electroplated onto a substrate. A saturation flux density (BS) of about 1.9 T (19 kG) is achieved by imposing a rotating external magnetic field during the electroplating process or during a subsequent annealing step. The boron is said to lower the coercivity (HC) of the pole elements to about 80 A/m (1.0 Oe) and an advantageously low anisotropic field (HK) of about 550 A/m (7 Oe) results from the electroplating process.
Continuing increases in media storage bit densities require continuing improvements in write head performance. It would be desirable to improve the saturation flux density (BS) of the simpler nickel-iron alloy pole layers by adding iron to the alloy to provide more than 62% iron by weight. The prior art generally teaches away from this idea because alloys with higher iron concentrations are known to have too much coercivity (HC=250 A/m or more) and thus cannot handle the high frequencies required to write high-density data to a high-HC medium. As a result of this widely-held belief, the pole material of choice in the art is currently nickel-iron alloy with about 55% iron by weight (Ni45Fe55). But the saturation flux density of this material is limited to about 1.75 T (17.5 kG) at best. Newer high-HC data recording media are pushing the flux limits for heads using this material. The few known alternative low-HC materials with higher BS are substantially more difficult to fabricate into acceptable pole structures.
These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.